Windows account limitation
The more you use Active Directory (AD), the more likely you find yourself the unhappy victim of one or more of the limits imposed either by AD itself or by an interface you use to administer it. The AD limits exist primarily to optimize AD's performance—they relate only marginally to the number of objects you can have.
For example, the maximum number of user accounts possible in a Windows NT domain was 40,000. The number of user accounts possible in an AD domain is huge. I don't know the absolute maximum, but Korea.com (an Internet portal providing a range of Web services to Korean citizens) has an 8-million-user-account AD implementation.
In another test, it says Windows 2000 Active Directory has been tested with over 30 million user accounts in a single domain ! Believe It.......